A friend (who shall now be known as Jeff, since he linked to me and has come out of the blogging shadows :-) asks an excellent question.
Only let your manner of life be worthy of the gospel of Christ, so that whether I come and see you or am absent, I may hear of you that you stand firm in one spirit, with one mind striving side by side for the faith of the gospel, and not frightened in anything by your opponents. This is a clear omen to them of their destruction, but of your salvation, and that from God. Phil 1:27-28
~ My question is how do you stay in one spirit when you don't believe a 100% with what your fellow believers think is true?
Here is a bit from Matthew Henry, that may shed some light (emphasis mine):
Three things he desired to hear of them; and they are all such as become the gospel:—1. It becomes those who profess the gospel to strive for it, to use a holy violence in taking the kingdom of heaven. The faith of the gospel is the doctrine of faith, or the religion of the gospel. There is that in the faith of the gospel which is worth striving for. If religion is worth any thing, it is worth every thing. There is much opposition, and there is need of striving. A man may sleep and go to hell; but he who will go to heaven must look about him and be diligent. 2. The unity and unanimity of Christians become the gospel: Strive together, not strive one with another; all of you must strive against the common adversary. One spirit and one mind become the gospel; for there is one Lord, one faith, one baptism. There may be a oneness of heart and affection among Christians, where there is diversity of judgment and apprehensions about many things. 3. Stedfastness becomes the gospel: Stand fast in one spirit, with one mind. Be stedfast and immovable by any opposition. It is a shame to religion when the professors of it are off and on, unfixed in their minds, and unstable as water; for they will never excel. Those who would strive for the faith of the gospel must stand firm to it.
I'm not sure of the exact answer, but I think Henry points us in the right direction. The unity is in relation to the external opposition. In other words, don't let differences among believers hinder our standing fast in the world. When it does hinder our standing fast, something must be done.